The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the removal of baseline error artifacts in images produced by ultrafast imaging methods.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this NMR signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of separate measurement cycles (referred to as "views") in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
A well known problem with MRI systems is the introduction of baseline errors into the received NMR echo signals. This error occurs when a dc level is added to the NMR echo signal during its reception, demodulation and digitization. This dc level may be introduced by dc offsets in the analog receiver circuitry, or by stray signals that are demodulated to a dc level along with the NMR echo signal. Continued improvements to the receiver hardware have reduced, but not eliminated this problem.
One way to eliminate baseline errors is to acquire two NMR echo signals at each phase encoding view. The phase of the RF excitation pulse is inverted 180.degree. for one of the two pulse sequences, and the two received NMR echo signals are subtracted. The subtraction nulls the dc level introduced by the receiver and doubles the level of the NMR signal for that view. Unfortunately, this solution also doubles the scan time because two measurements are required of each phase encoding view.
A similar solution which also addresses the increased scan time was disclosed in U.S. Pat. No. 4,612,504, issued Sep. 16, 1986 entitled "Method For Removing The Effects Of Baseline Error Components In NMR Imaging Applications". This method alternates the phase of the RF excitation pulse for successive phase encoding views. Prior to image reconstruction, the NMR signals for alternate phase encoding views are re-inverted so that the NMR signals in all views have the same polarity. This re-inversion also inverts any dc level in alternate views. As a result, the dc level alternates in polarity for successive views in k-space and is now a high frequency signal component in the phase encoding direction. The subsequent column Fourier transformation used during image reconstruction translates this high frequency component and places an artifact at the edges of the reconstructed image rather than its center. This is the prevailing method for solving the baseline error problem, and it is very effective.
The concept of acquiring NMR image data in a short time period has been known since 1977 when the echo-planar pulse sequence was proposed by Peter Mansfield (J. Phys. C.10: L55-L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a large number of NMR echo signals for each RF excitation pulse. These NMR signals can be separately phase encoded so that an entire scan of 64 views can be acquired in a single pulse sequence of 20 to 100 milliseconds in duration. The advantages of echo-planar imaging ("EPI") are well-known, and there has been a long felt need for apparatus and methods which will enable EPI to be practiced in a clinical setting. Other echo-planar pulse sequences are disclosed in U.S. Pat. Nos. 4,678,996; 4,733,188; 4,716,369; 4,355,282; 4,588,948 and 4,752,735.
A variant of the echo planar imaging method is the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al in an article in Magnetic Resonance in Medicine 3,823-833 (1986) entitled "RARE Imaging: A Fast Imaging Method for Clinical MR." The essential difference between the RARE sequence and the EPI sequence lies in the manner in which echo signals are produced. The RARE sequence utilizes RF refocused echoes generated from a Carr-Purcell-Meiboom-Gill sequence, while EPI methods employ gradient recalled echoes.
Yet another variant is described by D. A. Feinberg and K. Oshio "GRASE (Gradient and Spin Echo) MR Imaging: A New Fast Clinical Imaging Technique", in Radiology, 181:597-604, 1991.
All of these "ultrafast" imaging methods involve the acquisition of multiple echo signals from a single excitation pulse in which each acquired echo signal is separately phase encoded. Each pulse sequence, or "shot," therefore results in the acquisition of a plurality of views. In a single shot acquisition, the method previously used to eliminate image artifacts due to baseline errors cannot be used with these ultrafast pulse sequences, because there is no one-for-one correspondence between RF excitation pulse and NMR echo signal.